Resilience Assessment of Coral Reefs

Coral reefs and their associated seagrass beds and mangrove habitats support the highest marine biodiversity in the world. More than 500 million people worldwide depend on them for food, storm protection, jobs, and recreation. Their resources and services are worth an estimated 375 billion dollars each year, yet they cover less than one percent of the Earth’s surface. Unfortunately, many of the world’s coral reefs have been degraded, mainly due to human activities. According to the Status of Coral Reefs of the World: 2004, 70% of the worlds’ coral reefs are threatened or destroyed, 20% of those are damaged beyond repair, and within the Caribbean alone, many coral reefs have lost 80% of coral species.

Climate change is now recognized as one of the greatest threats to coral reefs worldwide. While a changing climate brings many challenges to coral reefs, one of the most serious and immediate threats is from mass coral bleaching associated with unusually high sea temperatures. Coral bleaching has lead to substantial damage to coral reefs on a global scale (16% of reefs suffered lasting damage in 1998 alone), with some areas losing 50-90% of their coral cover (Wilkinson 2000). Further degradation is predicted: severe coral bleaching events may be an annual occurrence by midcentury, even under optimistic climate scenarios (Hoegh-Guldberg 1999, Hughes et al. 2003). The amount of damage depends on not only the rate and extent of climate change, but also on the ability of coral reefs to cope with change. Importantly, the natural resilience of reefs, that maintains them in a coral dominated state, is being undermined by stresses associated with human activities on the water and on the land. Unmanaged, these stresses have the potential to act in synergy with climate change to functionally destroy many coral reefs and shift them to less diverse and productive states dominated by algae or suspension feeding invertebrates. Coral reefs are under pressure from a variety of human activities, including catchment uses that result in degraded water quality, unsustainable and destructive fishing, and coastal development. These local pressures act to reduce the resilience of the system, undermining its ability to cope with climate change, and lowering the threshold for the shift from coral-dominated phase to other phases. Increasingly, policy-makers, conservationists, scientists and the broader community are calling for management actions to restore and maintain the resilience of coral reefs to climate change, and thus avoid worst-case scenarios.

Two general properties determine the ability of coral communities to persist in the face of rising temperatures: their sensitivity and their recovery potential. Sensitivity relates to the ability of individual corals to experience exposure without bleaching, and if they bleach to survive. Recovery potential relates to the community’s capacity to maintain or recover its structure and function in spite of coral mortality. These properties at the coral colony and coral community level are termed ‘resistance’ and ‘resilience’, respectively (West and Salm 2003, Obura 2005, Grimsditch and Salm 2006). Together, they determine the resilience of coral communities to rising sea temperatures.